The power required for driving the cooling fans on high performance vehicles reduces the power available for propulsion and seriously limits vehicle performance. For maximum over-all efficiency the fan must be driven at the exact speed required to cool the engine and transmission fluids under each operating condition. Therefore it is desirable that the fan speed be controlled relative to the cooling demand and independent of engine and vehicle speed.
Various fan drive systems are known which provide variable ratio between the fan and the engine and thereby prevent the excessive waste of power at high speeds, however they have deficiencies which restrict vehicle performance under certain operating conditions, particularly during engine acceleration. In such systems, when the engine is accelerated to higher speed, the fan must be accelerated by the engine, and the fan inertia, reflected through the drive ratio, burdens the engine and reduces its acceleration rate.
The invention described in this disclosure presents an improved fan drive system that will provide fan speed as required by the cooling load and permit engine acceleration without the burden of fan inertia. The system consists of a power-take-off driven variable displacement hydraulic pump, which drives a hydraulic motor, which drives the fan. Motor and fan speed are controlled by varying the pump displacement. The control measures temperature of the fluid being cooled and governs fan speed relative to the fluid temperature. Fan speed is measured by the return flow from the motor. When the engine speed increases or decreases the control automatically decreases or increases pump stroke to hold the oil flow, motor speed and fan speed constant. Therefore, the engine can be accelerated without changing the fan speed or horsepower load. To improve engine acceleration, the control can be biased to reduce fan speed slightly as the engine speed increases and thereby reduce the load on the engine. This is accomplished by adding the make-up flow to the return flow from the fan motor before the speed measuring device; thus the control reduces drive pump stroke and motor speed to offset the increase in make-up flow as engine speed increases.